In order to understand the action of ultrasound on biological systems, studies on ultrasonic radiation of nucleic acids and their components are essential, especially in considering the increasing clinical use of ultrasound. Our systematic investigation has been conducted along a dual course: 1) the study of the sonochemical changes of nucleic acid bases, nucleosides, and nucleotides at low levels of sonication (less than or equal to 5 W per sq. cm, less than 30 min, 1 MHz) and 2) the examination of the inactivation of H. influenzae transformation DNA and the degradation of SV 40, phi times 174, and T7 DNA. About a year ago, modification of the continuous wave ultrasonicator made it possible to extend our studies to pulse mode. Our data suggest that 1) the extent of sonolysis is greatest at approximately 10 msec pulse width and decreases when the pulse width is either increased or decreased and 2) the extent of sonolysis exhibits a maximum at approximately 30 percent duty cycle when the percentage of duty cycle is either increased or decreased under our experimental conditions. In order to better understand these unexpected and provocative findings, we re-examined the fundamental aspects concerning the characteristics of ultrasonic radiation of aqueous systems with biological implications. In this study the threshold energies and kinetics of sonodegradation of thymine were investigated under different experimental conditions and explained on the basis of the "interphase diffusion" theory. The results show that in the limiting case of zero aeration rate, thymine degrades at 2.2 W per sq. cm. The degradation changes with the sonic energy due to the change in the number of cavitating bubbles (per unit volume per unit time), and the maximum radius of expansion bubbles. The results also show that degradation reaction takes place in the bubble-liquid interphase and the reaction kinetics change with the ambient temperature due to the competition between cavitation and substrate diffusion, which change conversely with respect to one another with a change in ambient temperature. During the coming year, we intend to use this additional knowledge to further re-examine the effects of variation of different parameters including dissolved inert gases, pH, temperature, etc. and to continue the identification of sonoproducts.